(1) Field of the Invention
The present invention relates to a mobile wireless packet data communication technique and, more particularly, to a wireless base station, a packet transfer apparatus, and a wireless communication system adapted to a packet data wireless communication system in which a data transmission rate dynamically changes in a radio channel.
(2) Description of the Related Art
In recent years, rapid growth of the Internet creates new needs for the high-speed wireless transmission technique. Awireless portable terminal performs not only speech communications but also, for example, communications of electronic mails, accesses to the Web, and increasingly uses an application via an IP network accompanying data transfer of a large amount such as distribution of music data and image data.
In the wireless data communications, when the status of a radio path deteriorates and a noise level becomes higher than a reception signal level, a burst bit error often occurs. When the status of the radio path is good and the reception signal level is relatively high, an error free state is achieved and no error occurs. Consequently, as one of means for realizing high-speed radio transmission, a method of performing a best-effort type communication by controlling parameters of modulation and an encoding system to optimum values while considering the degree of interference noise in a radio path has been proposed, for example, a method (hereinbelow, called a “1×EV method”) described by Paul Bender, Peter Black, Matthew Grob, Roberto Padovani, Nagabhushana Sindhushayana, and Andrew Viterbi, QUALCOMM, Incorporated “CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for Nomadic Users”, IEEE Communications Magazine, Vol. 38, pp. 70–77, July, 2000.
Connection on a radio path is, however, generally unstable as compared with connection on a wired IP network, and its transmission rate is generally low. Japanese Unexamined Patent Application No. 10-174185 describes that a wireless base station is provided with a buffer to absorb a difference in transmission rates in an interwork between an IP network and a radio network, and packet data to be transmitted to a mobile station is temporarily stored in the buffer.